Sensor arrangement for position sensing

ABSTRACT

A sensor arrangement for position sensing comprises a magnetic field source and a magnetoresistive element arranged in a magnetic field generated by the magnetic field source, which magnetoresistive element provides an output signal (R) dependent on a position (x) of the magnetoresistive element relative to the magnetic field source. A feedback controller is configured to receive the output signal (R) of the magnetoresistive element and is configured to adjust one or more of the position (x) of the magnetoresistive element relative to the magnetic field source and a strength of the magnetic field generated by the magnetic field source acting on the magnetoresistive element dependent on the output signal (R) of the magnetoresistive element.

FOREIGN PRIORITY

This application claims priority to GB Patent Application No. 1421183.3;filed Nov. 28, 2014 and all the benefits accruing therefrom under 35U.S.C. §119, the contents of which in its entirety are hereinincorporated by reference.

BACKGROUND

The present invention relates to a sensor arrangement for positionsensing and to a method for supporting determining the position of anobject.

High bandwidth, high resolution nanoscale sensing is a key enablingtechnology for nanoscale science and engineering. Application areasinclude life sciences, scanning probe microscopy, semiconductorfabrication and material science. Currently available position sensorsbased on optics, capacitors or inductive coils, although accurate andfast, do not scale down to micro-scales for use in micro-structures orin large-scale point-wise position sensing of macro-structures.Thermo-electric position sensors, on the other hand, scale down tomicro-scale, but suffer from low resolution and bandwidth.

A known position sensing concept is based on the property ofmagnetoresistance (MR). Magnetoresistance is the property an electricalresistance of a conductive layer sandwiched between ferromagnetic layerschanges as a function of a magnetic field applied to the layers. Amagnetoresistive sensor typically uses this property to sense themagnetic field.

BRIEF SUMMARY

According to an embodiment of one aspect of the invention, a sensorarrangement is provided for position sensing. The sensor arrangementcomprises a magnetic field source and a magnetoresistive elementarranged in a magnetic field generated by the magnetic field source. Themagnetoresistive element provides an output signal dependent on aposition of the magnetoresistive element relative to the magnetic fieldsource. A feedback controller is provided and configured to receive theoutput signal of the magnetoresistive element and to adjust one or moreof the position of the magnetoresistive element relative to the magneticfield source and a strength of the magnetic field generated by themagnetic field source and acting on the magnetoresistive elementdependent on the output signal of the magnetoresistive element.

According to another embodiment, a method for supporting determining theposition of an object is provided. The method includes coupling one of amagnetic field source and a magnetoresistive element with the object andarranging the magnetoresistive element in a magnetic field generated bythe magnetic field source. The method also includes receiving an outputsignal (R) from the magnetoresistive element dependent on its position(x) relative to the magnetic field source and adjusting one or more ofthe position (x) of the magnetoresistive element relative to themagnetic field source and a strength of the magnetic field generated bythe magnetic field source acting on the magnetoresistive elementdependent on the output signal (R) of the magnetoresistive element.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its embodiments will be more fully appreciated byreference to the following detailed description of exemplary embodimentsin accordance with the present invention when taken in conjunction withthe accompanying drawings. The figures are illustrating:

FIG. 1 a schematic cut of a sensor as used in a sensor arrangementaccording to an embodiment of the present invention;

FIG. 2 diagrams illustrating an impact of a magnetic field on amagnetoresistive element according to an embodiment of the presentinvention;

FIG. 3 a diagram illustrating an impact of a magnetic field on amagnetoresistive element according to an embodiment of the presentinvention;

FIG. 4 a block diagram of a sensor arrangement according to anembodiment of the present invention;

FIG. 5 a block diagram of a device including a sensor arrangementaccording to another embodiment of the present invention;

FIG. 6 a cut of a sensor as used in a sensor arrangement according to anembodiment of the present invention representing a first actuation;

FIG. 7 a cut of a sensor as used in a sensor arrangement according to anembodiment of the present invention representing another actuation;

FIG. 8 a block diagram of a sensor arrangement according to anembodiment of the present invention;

FIG. 9 a corresponding measurement characteristic;

FIG. 10 a cut of a sensor as used in a sensor arrangement according toan embodiment of the present invention representing a further actuation;

FIG. 11 a block diagram of a sensor arrangement according to anotherembodiment of the present invention; and

FIG. 12 a corresponding measurement characteristic.

DETAILED DESCRIPTION

As an introduction to the following description, it is first pointed ata general aspect of the invention concerning a sensor arrangement basedon the magnetoresistive effect.

A magnetoresistive sensor preferably comprises a magnetoresistiveelement comprising a stack of layers which stack of layers includes atleast a conductive layer in between two magnetic layers, and preferablyin between two ferromagnetic layers. Specifically, the magnetoresistiveelement is a giant magnetoresistive element based on the underlyingeffect wherein in a layer stack comprising a non-magnetic conductivelayer sandwiched between two ferromagnetic layers a change in theelectrical resistance can be observed subject to a magnetizationorientation in the ferromagnetic layers. In the absence of an externalmagnetic field, the magnetization orientation of the correspondingferromagnetic layers is antiparallel. By means of applying an externalmagnetic field to at least one of the ferromagnetic layers the subjectmagnetization orientation is changed which in turn leads to themagnetization orientation in the two ferromagnetic layers becomesparallel which in turn causes the electrical resistance of the layerstack to change. The electrical resistance of the layer stack is alsodenoted as electrical resistance of the magnetoresistive element. Thechange in the electrical resistance may be monitored and taken as ameasure for the presence of an external magnetic field applied. Whensuch external magnetic field is generated by a magnetic field sourceattached to an object, a position of such object may be determined withrespect to the magnetoresistive element. Specifically, a significantchange of the electrical resistance in the conductive layer can beobserved when the magnetization orientation of the ferromagnetic layersis changed between a parallel alignment to an antiparallel alignment orvice versa. In a parallel alignment of the magnetization orientation ofthe ferromagnetic layers the electrical resistance in the conductivelayer is rather low while in an antiparallel alignment of themagnetization orientation of the ferromagnetic layers the electricalresistance in the conductive layer is rather high. The change inconductivity is based on spin-dependent interfacial electron scattering.Electrons passing the magnetoresistive element may show a short meanfree path at antiparallel magnetization orientation in the ferromagneticlayers owed to an increased spin dependent electron scattering at theinterfaces between the conductive layer and the ferromagnetic layerswhile electrons may show a longer mean free main path owed to a lessspin dependent interfacial electron scattering when the ferromagneticlayers show a parallel magnetization orientation.

The magnetoresistive element hence preferably comprises a stack oflayers which at least includes one conductive layer in between twoferromagnetic layers, but which stack of layers may include multipleconductive layers sandwiched between adjacent ferromagnetic layers,wherein an overall electrical resistance of the stack of layers mayfinally be measured and allow for a determination of the position of thefield generating magnet relative to the magnetoresistive element. Anexternal magnetic field applied may cause the magnetization orientationin these ferromagnetic layers to switch from an antiparallel alignmentto a parallel alignment or vice versa. Generally, and specifically forthe present embodiments of the invention, the magnetization orientationof both ferromagnetic layers may be floating or, alternatively, themagnetization orientation of one of the ferromagnetic layers may bepinned such that its magnetization orientation may not change even underthe application of an external magnetic field. The external magneticfield may then only act on the other ferromagnetic layer and cause itsmagnetization orientation to change subject to a position the magnettakes.

In the context of the present embodiments of the invention, thefollowing geometrical definitions are used: The layers of the layerstack may have a longitudinal extension along a longitudinal axis and alateral extension along a transverse axis. Accordingly, a height of thestack extends orthogonal to a plane defined by the longitudinal and thetransverse axis along a vertical axis. A sensor central axis of themagnetoresistive element is defined as an axis along the vertical axis,and at half of the longitudinal extension of the layers. It ispreferably assumed that a magnetic field source and/or an objectcomprising the magnetic field source is at least movable along thelongitudinal axis, which means that the magnetic field source is movablealong the longitudinal extension of the layers of the layer stack.Preferably, the magnetic field source is arranged at a vertical distanceD>0 from the magnetoresistive element such that the magnet field sourceand an upper surface of the magnetoresistive element are spaced apartalong the vertical axis by the distance D. Moreover, the magnetic fieldsource provides a magnetic field approximating a magnetic dipoleincluding a North (N) and a South (S) pole. A dipole axis of themagnetic dipole is assumed to be a straight connection between the Npole and the S pole.

In conventional magnetoresistive position sensing concepts a dipole axisof the magnetic dipole of the magnet is aligned in parallel to alongitudinal extension of the layers of the layer stack contributing tothe magnetoresistive element. Given that a gradient of the magneticfield is responsible for generating a change in the magnetizationorientation of the one or more ferromagnetic layers, it was observedthat in the conventional concept the gradient of the magnetic fieldalong the longitudinal extension of the layers of the stack is ratherlow. The gradient is defined as a change in magnetic flux for a smallvariation in the location x. However, the higher the gradient of themagnetic field is along the longitudinal extension of the layers of thestack the higher the sensitivity/resolution of the sensing scheme isgiven that only a small variation of the position of the magnet maycause an impact on the magnetization orientation of the ferromagneticlayers because such small variation in position still leads to asubstantial change in magnetic flux owed to the high gradient. Inaddition, it was observed that for typical magnet geometries such asrectangular permanent magnets, an absolute strength of the magneticfield at a given distance from the magnet increases with the gradient.Because there is an upper bound on the strength of the magnetic fieldafter which the magnetoresistive element saturates, the gradient of themagnetic field that can effectively be used for sensing is limited.

According to an embodiment of the present invention, the dipole axis ofthe magnetic field source is aligned orthogonal to the plane defined bythe longitudinal axis and the transverse axis. Hence, the dipole axis isalso orthogonal to a sensing direction of the magnetoresistive elementwhich sensing direction is defined by the longitudinal extension of thelayers of the stack in direction of the longitudinal axis. Orthogonal inthis context shall include tolerances of +/−20° degrees, i.e. the dipoleaxis is supposed to be arranged with a range of 70° to 110° degrees withrespect to the plane defined by the longitudinal axis and the transverseaxis.

In one embodiment of the present invention, the magnetic field source isa permanent magnet of a size comparable to the size of the stack oflayers. This includes a width of the permanent magnet along thelongitudinal axis comparable to the longitudinal extension of the layersof the stack. In this context, comparability preferably includes a widthof the permanent magnet not more than ten times the longitudinalextension of the layers of the stack, and preferably not less than halfof the longitudinal extension of the layers of the stack. In a verypreferred embodiment, the magnet is of rectangular shape in a planedefined by the longitudinal axis and the vertical axis, and in anothervery preferred embodiment is of cubic shape in its three dimensions.According to another embodiment of the present invention, the magneticfield source is embodied as electromagnet.

FIG. 1 illustrates a schematic side cut of sensor arrangement accordingto an embodiment of the present invention comprising a magnetoresistiveelement 1 and a magnetic field source 2. The magnetoresistive element 1is an element specifically underlying the giant magnetoresistivityphenomenon, wherein in thin magnetic multilayers with one or moreconductive layers in between spin coupling occurs. The presentmagnetoresistive element 1 comprises a layer stack 11 arranged on asubstrate 12 which layer stack 11 includes at least a firstferromagnetic layer 111, a conductive and non-magnetic layer 112, asecond ferromagnetic layer 113, and an antiferromagnetic cover layer114. Magnetic moments of the ferromagnetic layers 111 and 113 arenaturally aligned antiparallel with respect to each other when noexternal magnetic field is applied. In case an external magnetic fieldof sufficient strength is applied, magnetic moments become parallel inthe ferromagnetic layers 111 and 113, i.e. the magnetizationorientations in the ferromagnetic layers 111 and 113 are aligned inparallel. An antiparallel magnetization orientation in the ferromagneticlayers 111 and 113 result in a rather low mean free path of electronspassing the layer stack 11 leading to a rather high electricalresistance in the layer stack 11. On the other hand, a parallelmagnetization orientation in the ferromagnetic layers 111 and 113 resultin a rather high mean free path of electrons passing the layer stack 11leading to a rather low electrical resistance in the layer stack 11.This effect is based on the dependence of electron scattering on thespin orientation at the interfaces of the layers 111, 112 and 113.

The layers of the layer stack show a longitudinal extension L along thelongitudinal axis X. The layers also extend along the transverse axis Yinto the plane of projection. A current I may be applied to the layerstack during position sensing. The layers of the stacks are arrangedvertically, i.e. along vertical axis Z on top of each other.

The magnetic field source 2 may in the present embodiment be a permanentmagnet, or in another embodiment an electromagnet, and more specificallyan electromagnet comprising a coil and a magnetic core, or a solenoid.Preferably, miniature ultra-thin magnetic dipoles may be used forenabling the stack of layers being exposed to magnetic fields with avery high gradient. A position sensing resolution of less than <200 pmover 100 kHz may be achieved. Presently, the magnetic field source 2 isarranged at a distance D from a surface of the magnetoresistive element1. The magnetic field source 2 presently is of rectangular shape with awidth W along the longitudinal axis X, a depth not visible along thetransverse axis y, and a height H along the vertical axis Z.

In the present position, a dipole axis DA of the magnetic field source 2coincides with a sensor central axis SA of the magnetoresistive element1. This means, the magnetic field source 2 is centered above themagnetoresistive element 1. The magnetic field source 2 provides amagnetic field illustrated in dashed lines which virtually may beseparated into a first portion B1 and a second portion B2. As can bederived from FIG. 1, magnetic field lines from both portions B1 and B2may affect the layer stack and as such impact the magnetizationorientation of the ferromagnetic layers 113 and/or 111. Both portions B1and B2 qualify by a high gradient when meeting the magnetoresistiveelement 1 which is desired for achieving a high resolution because yetsmall changes in the position of the corresponding magnetic field source2 may cause a realignment of the magnetic spin in the ferromagneticlayers 111, 113 which in turn may affect the conductivity of the layerstack.

Hence, while in conventional position sensor arrangements the magneticfield source is aligned with its dipole axis DA in parallel to a sensingdirection X of the magnetoresistive element 1 coinciding with thelongitudinal extension of the layers, in the present embodiment themagnet field source 2 is aligned with its dipole axis DA orthogonal tothe sensing direction X of the magnetoresistive element 1. For thisreason, both a high gradient and a low strength of the magnetic fieldcan be achieved at the same time. The low strength of the magnetic fieldis owed to the fact that the magnetic field lines go through zeromagnitude because the magnetic field in the subject portion whenprojected to the sensing direction changes direction.

In FIG. 1, another scenario is indicated by a dotted dipole axis DA′when the magnetic field source 2 is offset from the center axis SA ofthe magnetoresistive element 1 in direction of the longitudinal axis Xby a distance F. This may, for example, be the case when an object themagnetic field source 2 is attached to is moved with respect to themagnetoresistive element 1.

FIG. 2 shows diagrams illustrating an impact of a magnetic field on amagnetoresistive element according to an embodiment of the presentinvention. In diagram 2 a) the magnetic field lines of a magnetic dipoleare shown wherein the longitudinal extension of the magnetoresistiveelement is indicated by line B-B which magnetoresistive element may bepositioned at various distances D from the magnet as is indicated by thedouble arrow. The magnetic field lines are shown in a plane defined bythe longitudinal axis X and the vertical axis Z. In diagram 2 b), acorresponding flux density—also denoted as strength of the magneticfield—is shown over the longitudinal axis X, and specifically is shownfor different distances D between the magnet and the magnetoresistiveelement. It can be derived from diagram 2 b) that the magnetic flux iszero at the location of the dipole axis X=XD while it is non-zero for Xoutside XD. In diagram 2 c), the corresponding gradient of the magneticfield is shown over the longitudinal axis X, and specifically is shownfor different distances D between the magnet and the magnetoresistiveelement with D1>D2. It can be derived from diagram 2 c), that at thelocation of the dipole axis X=XD, the gradient has a maximum value whileoutside the dipole axis X=XD the gradient is lower in its absolutevalue. It further can be derived from diagram 2 b) that the bigger thedistance D is the lower the magnetic flux density is while the closerthe magnetoresistive element is arranged with respect to the magnet thehigher the magnetic flux density is. On the other hand, it can bederived from diagram 2 c) that the closer the magnetoresistive elementgets to the magnet, the higher the maximum gradient value becomes atX=XD.

In terms of resolution a high gradient and a low flux density is desiredat the same time. Diagram 2 c) in addition shows that a determination ofthe distance D may preferably also take into account the linearity ofthe gradient: This sensing range can be impacted by the magnetic field.It may be desired at the same time to provide a position sensor sensingwith a rather linear property over the sensing range. However, fromdiagram 2 c) it can be derived that the closer the magnetoresistiveelement is arranged with respect to the magnet, the higher the maximumgradient becomes, but after a point the gradient becomes less linearacross the sensing range. Hence, it may be preferred to select adistance D12 that at the same time provides a sufficient gradient and asufficient linearity in the sensing range.

FIG. 3 depicts a diagram illustrating an impact of a magnetic field on amagnetoresistive element according to an embodiment of the presentinvention. It is assumed that in the x=0 longitudinal position themagnetic dipole is arranged centered on the magnetoresistive element 1such as shown in FIG. 1, i.e. the dipole axis DA coincides with thesensor axis SA. As can be derived from FIG. 3, in a region to the leftand right of x=0, there is a linear relation between the change inresistance and x. However, at some longitudinal position x magneticsaturation occurs such that the change in resistance is not indicativeof the longitudinal position x. However, it is only a rather shortsensing range in x, where linearity is given. Generally, the termsensing range is defined as the range in dimension X where no saturationoccurs.

On the other hand, the diagram in FIG. 3 shows the effect of varying thevertical distance between the magnetoresistive element and the magneticdipole: It can be derived that for short vertical distances a highsensitivity/slope can be achieved while for longer vertical distancesthe sensitivity drops. On the other hand, for longer vertical distances,the sensing range in x broadens, and saturation does not even occur. Themagnetic dipole may be positioned relative to the magnetoresistiveelement at a vertical distance such that a large gradient of the appliedmagnetic field is achieved in the sensing range while at the same timegood linearity properties over the sensing range are provided. Sucharrangement may lead to a high bandwidth sensing but on the other handto a limited sensing range owed to the magnetic saturation.

Based on the insight of the characteristics of such a sensor asillustrated in FIG. 3, it is suggested to adapt operating parameters ofthe sensor, i.e. parameters for operating the magnetic field sourceand/or the magnetoresistive element in order to broaden the sensingrange without running into saturation.

Under the assumption, that according to FIG. 3 the present operatingscenario is in the magnetic saturation, e.g. exactly where the assigned“magnetic saturation” arrow points at, there may be several measures toreconfigure the operation of sensor to a non-saturated region forfurther operations. In one embodiment, the vertical distance between themagnetic field source and the magnetoresistive element may be adjusted,and in this particular example may be increased according to the dottedarrow. This results in sensing in a non-saturated region however, at alower sensitivity compared to the previous sensing region when stillbeing in the non-saturated state. In a different embodiment, the sameeffect can be achieved by adjusting the operating current of anelectromagnet in case the magnetic field source is embodied as anelectromagnet. In a different approach, which is indicated by a dashedline arrow, the sensing region is adjusted by bringing the dipole axisand the sensor axis closer together again, i.e. by a relative movementof the magnetic dipole and the magnetoresistive element in sensingdirection, i.e. in x-direction. For any of the introducedre-adjustments, the output signal of the magnetoresistive element isused in a feedback loop.

Hence, new sensing methods are introduced in which feedback is used toenhance the range of operation and/or position sensitivity of thesensor. In doing so, the specific properties of a magnetoresistiveposition sensing concept are exploited and materialized in embodiments,such as the possibility of adjusting the magnetic field that affects thesensing element in two different ways at the same time, e.g. by changingthe strength of the magnetic field and/or by changing the position ofthe magnetic field source. This idea allows for the development of alarge-range, high-resolution magnetoresistive position sensorarrangement.

FIG. 4 illustrates a block diagram of a sensor arrangement according toan embodiment of the present invention. A magnetic field source 2 actson a magnetoresistive element 1, for example such as disclosed in any ofthe previous embodiments. The magnetoresistive element 1 provides anoutput signal R which presently reflects a resistance of themagnetoresistive element 1. This output signal R depends on the relativeposition between the magnetoresistive element 1 and the magnetic fieldsource 2 along the longitudinal axis X. The relative position betweenthese elements may be the one an object, that preferably is movablealong the longitudinal axis X, to which one of the magnetoresistiveelement and the magnetic field source is coupled or attached. Any suchmovement of the object is referred to by x′.

The output signal R of the sensor 1, 2 is supplied to a feedbackcontroller 3 which in addition receives a reference output R0. Thefeedback controller 3 is configured to control one or more actuators 6,7, or the magnetic field source 7, such that the output signal R ismaintained equal to the reference output R0, which may in one embodimentrepresent a constant resistance value, to which the output signal Rshall be controlled to. In another embodiment, the reference output isvariable. For this purpose, it is preferred that a deviation of theoutput signal R from the reference output R0 is determined by means ofsubtraction. The deviation may be converted in one or more controlsignals ux, uz, uB for adjusting the operation and specifically foradjusting the sensing range. uB represents a control signal foradjusting a magnetic flux B of the magnetic field source 2 that meetsthe magnetoresistive element 1, such as a current to a coil of themagnetic field source 2. ux denotes a control signal for adjusting therelative position x between the magnetoresistive element 1 and themagnetic field source 2 along the longitudinal axis X. This distance maybe adjusted by the suitable actuator 6 for longitudinally moving eitherthe magnetoresistive element 1 or the magnetic field source 2 inresponse to the control signal ux. uz denotes a control signal foradjusting the relative position z between the magnetoresistive element 1and the magnetic field source 2 along the vertical axis Z. This distancemay be adjusted by the suitable actuator 7 for longitudinally movingeither the magnetoresistive element 1 or the magnetic field source 2 inresponse to the control signal uz. In case of a deviation between theoutput signal R and the reference output R0, the relative positionbetween the magnetoresistive element 1 and the magnetic field source 2can be adjusted in x and/or in z direction, and/or the strength of themagnetic field as acting on the magnetoresistive element 1 can beadjusted dependent to the determined deviation. It is noted that thedeviation may be converted in either the control signals ux, uz, uB orinto two or all three of the control signals ux, uz, uB.

FIG. 5 illustrates a diagram of a sensor arrangement according to anembodiment of the present invention. The right hand part reflects thepieces of the sensor arrangement that are also depicted in FIG. 4. x′refers to the position of an object 5, which may be a tool in a plant,or any other object the position of which is desired to be known and/orcontrolled. Presently, the position of the object 5 is controlled by acontroller 4 to meet a reference signal. Therefore, the present positionof the object 5, which is represented by the measurement signal suppliedby the feedback controller 3 is compared to the reference signal whichrepresents the desired position. The measurement signal preferably isderived from the one or more control signals ux, uz, uB and refers tothe present position of the object 5. For example, in case there is noadjustment x of the longitudinal position and no adjustment z of thevertical position for sensitivity purposes, the measurement signal candirectly be derived from the magnetic field control signal uB.

FIG. 6 illustrates a cut of a sensor 1, 2 according to an embodiment ofthe present invention representing a first actuation. The sensorcomprises a magnetic field source 2 in form of a solenoid comprising atleast one coil 21 for generating a magnetic field as indicated by thearrows. A strength of the magnetic field is dependent on a current Ithrough the coil 21 as is shown by:

$B_{\rho} = {\frac{\mu_{0}I}{2\;\pi}\frac{1}{L}{\sqrt{\frac{a}{\rho}}\left\lbrack {{\frac{k^{2} - 2}{k}{K\left( k^{2} \right)}} + {\frac{2}{k}{E\left( k^{2} \right)}}} \right\rbrack}_{\zeta -}^{\zeta +}}$$B_{z} = {\frac{\mu_{0}I}{4\;\pi}\frac{1}{L}{\frac{1}{\sqrt{a\;\rho}}\left\lbrack {\zeta\;{k\left( {{K\left( k^{2} \right)} + {\frac{a - \rho}{a + \rho}{\Pi\left( {h^{2},k^{2}} \right)}}} \right)}} \right\rbrack}_{\zeta -}^{\zeta +}}$

Hence, by modulating the current I through the coil 21, the strength ofthe magnetic field, and therefore the strength of the magnetic field asit meets a magnetoresistive element 1 which in the present embodiment isarranged on a substrate 12 or other frame of reference, can be modulatedprecisely and at high speeds. In this embodiment, the relative positionbetween the magnetoresistive element 1 and the magnetic field source 2is not touched for increasing sensitivity.

FIG. 7 illustrates a cut of a sensor 1, 2 according to an embodiment ofthe present invention representing a different actuation. The sensorcomprises a magnetic field source 2, such as a permanent magnet or anelectromagnet, for generating a magnetic field as indicated. A strengthof the magnetic field as meeting the magnetoresistive element 1 is alsodependent on and in particular proportional to a vertical distance zbetween the magnetoresistive element 1 and the magnetic field source 2as is approximated for a magnetic dipole as follows:B _(x)=3Mxz/r ⁵B _(x)=3Myz/r ⁵B _(z) =M(3z ² −r ²)r ⁵

Hence, an actuator 7 is provided, e.g. a piezo-actuator, for adjusting avertical distance z between the magnetic field source 2 and themagnetoresistive element 1 along the vertical axis Z. By means ofcontrolling the actuator 13 and adjusting the vertical distance, thestrength of the magnetic field at the magnetoresistive element 1 can bemodulated.

FIG. 8 illustrates a block diagram of a sensor arrangement according toan embodiment of the present invention. This sensor arrangement differsfrom the sensor arrangement of FIG. 4 in that only the strength B of themagnetic field is modified subject to a deviation of the output signal Rfrom the reference output R0. The corresponding adjustment of thestrength of the magnetic field again is referred to by uB. Hence, theresistance of the magnetoresistive element 1 is kept constant in thepresence of position changes by modulating the strength of the magneticfield. Position changes refer to the relative position between themagnetoresistive element and the magnetic field source in longitudinaldirection x′ and may be considered—if not induced on purpose—asdisturbances which are leveled out by the proposed method. Themeasurement signal provided in FIG. 8 is derived from the control signaluB converted into a position value. The adjustment of the magnetic fieldstrength can be effected by any one of the concepts addressed in FIGS. 6and 7.

FIG. 9 illustrates a measurement characteristic corresponding to thesensor arrangement of FIG. 8. For an open-loop sensing without thefeedback controller 3, the measurement signal over the longitudinalposition of the magnetoresistive element 1 is depicted by a dotted line.Instead, the measurement signal over the longitudinal position of themagnetoresistive element 1 for the proposed closed loop controlaccording to FIG. 8 is depicted in a straight line. It can be derivedthat the open loop sensing enters into magnetic saturation at aboutx˜+/−0.03 μm. As a result, positions x outside ˜+/−0.03 μm cannot besensed, i.e. cannot be discriminated from each other. In contrast, whenthe sensor 1, 2 is arranged in a closed feedback loop sensorarrangement, there is no saturation owed to the feedback control whichpersistently moves the operation point of the sensor 1, 2 into asaturated region of sensing.

FIG. 10 illustrates a cut of a sensor according to an embodiment of thepresent invention representing a different actuation. The sensorcomprises a magnetic field source 2, such as a permanent magnet or anelectromagnet, for generating a magnetic field as indicated. Anotheractuator 6 is provided, e.g. a piezo-actuator, for adjusting therelative position between the magnetic field source 2 and themagnetoresistive element 1 along the longitudinal axis X, and presentlyacts on the magnetoresistive element 1.

FIG. 11 illustrates a block diagram of a sensor arrangement according toanother embodiment of the present invention. This sensor arrangementdiffers from the sensor arrangement of FIG. 4 in that it is only therelative position between the magnetoresistive element 1 and themagnetic field source 2 along the longitudinal axis X that is modifiedsubject to a deviation of the output signal R from the reference outputR0. The corresponding control signal for adjusting the position betweenthe magnetoresistive element 1 and the magnetic field source 2 isreferred to by ux. Hence, the resistance of the magnetoresistive element1 is kept constant at the value represented by R0 in the presence ofposition changes by modulating the longitudinal position x. Themeasurement signal provided in FIG. 11 is derived from the controlsignal ux converted into a position value. The adjustment of thelongitudinal position can be effected by sensor according to FIG. 10,for example.

FIG. 12 illustrates a measurement characteristic corresponding to thesensor arrangement of FIG. 11. For an open-loop sensing without thefeedback controller 3, the measurement signal over the longitudinalposition of the magnetoresistive element 1 is depicted by a dotted line.Instead, the measurement signal over the longitudinal position x of themagnetoresistive element 1 for the proposed closed loop controlaccording to FIG. 11 is depicted in a straight line. It can be seen thatthe open loop sensing enters into magnetic saturation at about x˜+/−0.03μm. As a result, positions x outside ˜+/−0.03 μm cannot be sensed, i.e.cannot be discriminated from each other. In contrast, when the sensor 1,2 is arranged in a closed feedback loop sensor arrangement, there is nosaturation owed to the feedback control applied which persistently movesthe operation point of the sensor 1, 2 into a non-saturated and linearregion of sensing.

Embodiments of the present invention may be applicable to positionsensing in industry, to lithography, and specifically in connection withsemiconductors such as the manufacturing, EDA and testing ofsemiconducting devices, and specifically of micro- and/ornano-electromechanical devices. Any such position sensor may transmitits result wire bound or wireless to an evaluation unit.

What is claimed is:
 1. A sensor arrangement for position sensing,comprising a magnetic field source, a magnetoresistive element arrangedin a magnetic field generated by the magnetic field source and providingan output signal (R) dependent on a position (x) of the magnetoresistiveelement relative to the magnetic field source, and a feedback controllerconfigured to receive the output signal (R) of the magnetoresistiveelement and to adjust one or more of the position (x) of themagnetoresistive element relative to the magnetic field source and astrength of the magnetic field generated by the magnetic field sourceacting on the magnetoresistive element dependent on the output signal(R) of the magnetoresistive element, wherein the magnetic field sourcecomprises an electromagnet for generating the magnetic field dependenton an electrical current applied to the electromagnet, and wherein thefeedback controller is configured to adjust the strength of the magneticfield acting on the magnetoresistive element by controlling theelectrical current supplied to the electromagnet.
 2. The sensorarrangement of claim 1, wherein one of the magnetoresistive element andthe magnetic field source is coupled to an object, wherein the feedbackcontroller is configured to determine a position of the object dependenton one or more of: an adjustment of the strength of the magnetic fieldgenerated by the magnetic field source; the adjustment of the position(x) of the magnetoresistive element relative to the magnetic fieldsource; and the output signal (R).
 3. The sensor arrangement of claim 2,wherein the magnetoresistive element comprises a stack of layersincluding at least a conductive layer in between two magnetic layerswhich layers have a longitudinal extension (L) along a longitudinal axis(X) and a lateral extension along a transverse axis (Y), wherein themagnetic field source has a dipole axis (DA) extending along a verticalaxis (Z) orthogonal to a plane defined by the longitudinal axis (X) andthe transverse axis (Y), and wherein an electrical resistance of themagnetoresistive element depends on a position of the magnet fieldsource along the longitudinal axis (X).
 4. The sensor arrangement ofclaim 3, wherein the object (5) is movable along the longitudinal axis(X) a position of which object (5) along the longitudinal axis (X) is tobe sensed by the sensor arrangement.
 5. The sensor arrangement of 4,further comprising an actuator for moving one or more of the magneticfield source and the magnetoresistive element relative to each otheralong the longitudinal axis (X), wherein the feedback controller isconfigured to adjust the position (x) of the magnetoresistive elementalong the longitudinal axis (X) relative to the magnetic field source bycontrolling the actuator.
 6. The sensor arrangement of claim 5, whereinthe feedback controller is configured to adjust the position (x) of themagnetoresistive element relative to the magnetic field source dependenton a deviation of the output signal (R) of the magnetoresistive elementfrom a reference output (R₀).
 7. The sensor arrangement of claim 6,further comprising another actuator for moving one or more of themagnetic field source and the magnetoresistive element along thevertical axis (Z), wherein the feedback controller is configured toadjust a vertical position (z) of the magnetoresistive element along thevertical axis (Z) relative to the magnetic field source by controllingthe other actuator, and wherein the feedback controller is configured toadjust the strength of the magnetic field acting on the magnetoresistiveelement by controlling the other actuator.
 8. The sensor arrangement ofclaim 7, wherein the feedback controller is configured to adjust thestrength of the magnetic field acting on the magnetoresistive elementdependent on a deviation of the output signal (R) of themagnetoresistive element from a reference output (R₀).
 9. The sensorarrangement of claim 8, wherein the feedback controller is configured toadjust the strength of the magnetic field acting on the magnetoresistiveelement dependent on a deviation of the output signal (R) of themagnetoresistive element from a reference output (R₀).
 10. The sensorarrangement of claim 9, wherein the output signal (R) represents aresistance of the magnetoresistive element, wherein the reference output(R₀) represents a constant reference resistance value, and wherein thefeedback controller is configured to adapt the one or more of theposition of the magnetoresistive element relative to the magnetic fieldsource and the strength of the magnetic field of the magnetic fieldsource to maintain the resistance of the magnetoresistive element equalto the constant reference resistance value.
 11. The sensor arrangementof claim 10, wherein the reference output (R₀) is selected from a rangeof output signal values in which the combination of the magnetoresistiveelement and the magnetic field source are operated in a non-saturatedstate.
 12. The sensor arrangement of claim 11, wherein the magneticfield source is arranged at a distance D>0 μm from the magnetoresistiveelement along the vertical axis (Z).